One of the most remarkable features of Apicomplexan parasites is their ability to infect another eukaryotic cell and co-opt the functions of such a cell once inside. For many years, Toxoplasma gondii has served as an important and informative model for dissecting these processes. The act of invasion is associated with injection into the host cell of key components of the apical secretory organelles known as rhoptries. These organelles contain many parts of the invasion machinery itself, located within the rhoptry necks and therefore known as RONs, as well as an assortment of effector proteins known as ROPs that are located within the rhoptry bulbs and serve to co-opt host cell functions. Much work by us and others has revealed the identity of many of these RON and ROP proteins but the function of only a few has emerged from biochemical and genetic studies and nothing is known about how they are introduced. The goal of this current application is to determine the mechanism of how rhoptries introduce their contents into the host cell and how this injection sets up the infection in its earliest stages in vivo. To accomplish these goals, we will use innovative approaches involving genetic screens with specifically engineered parasites and in vivo studies with specifically engineered reporter mice. The identification of the injection apparatus, which appears to be conserved between Toxoplasma and its important cousin, Plasmodium, will allow novel strategies for interrupting this crucial step in invasion. Similarly, he determination of the first changes that occur within the intestinal epithelium of animals acquiring an oral infection with encysted bradyzoites will reveal how Toxoplasma has evolved to establish the infection in its earliest stages. This crucial interaction between the parasite and host is pivotal but no method has previously existed to explore it in vivo. The work we propose here will, therefore, address two important, unanswered questions.